The present invention relates generally to systems for measuring gas concentrations of chemical species. More particularly, the present invention relates to systems to measure the presence of trace ammonia gas in the presence of large carbon dioxide and water vapor populations or to measure any combination of ammonia, carbon dioxide and water simultaneously.
Most applications for which measurement of trace ammonia concentration is pertinent include high background levels of carbon dioxide and water vapor that can potentially interfere with the measurement. These applications include, but are not limited to, industrial chiller plants, where ammonia leaks that result in a few parts-per-million (ppm) concentration of ammonia in an atmospheric background with approximately 400 ppm carbon dioxide and 2 percent of water need to be detected; car exhaust analysis, for which ppm or less concentrations of ammonia need to be detected in the presence of 5-15% carbon dioxide and 5-20% water vapor; breath analysis, for which sub-ppm concentrations of ammonia need to be detected in the presence of approximately 2-6% carbon dioxide and 5-10% water vapor; and a variety of industrial process systems for which small amounts of ammonia gas need to be detected in the presence of other gases, such as within semiconductor fabrication facilities.
Current systems utilize various different techniques such as, for instance, electrochemical sensors, mass spectroscopy, or chemiluminescence. Many devices, such as electrochemical sensors, are sensitive to interfering species, and thus are not suitable. Mass spectroscopy is a slow, bulky and expensive process. Chemiluminescence, which is sensitive and expensive, actually detects the presence of ammonia via chemical conversion of any species containing nitrogen, and is thus not a direct or absolute measurement. Optical techniques such as laser spectroscopic sensors are advantageous over electrochemical sensors, mass spectroscopy or chemiluminescence because optical techniques could make absolute measurements very quickly, using affordable off-the-shelf components, and without calibration requirements or cross-sensitivity from other species. However, current optical techniques used to measure ammonia are susceptible to interfering absorption of laser radiation by carbon dioxide. This occurs, for example, at many wavelengths near 2 xcexcm, 1.5 xcexcm or near 10 xcexcm. Mihalcea et al. (in a paper entitled diode laser absorption measurements of CO2, H2O, N2O and NH3 near 2.0 micrometer and published in Appl. Phys. 67:283:288, 1998) demonstrated that a laser could be used with a wavelength near 2 xcexcm to measure different species. However, Mihalcea et al. did not teach or anticipate the fundamental parameters, such as the wavelength, transitions or pressure, necessary to measure ammonia substantially free from interference of other species. As a person of average skill in the art would readily appreciate, it would be difficult to determine which parameters work best or are optimal to measure ammonia substantially free from interference of other species. Accordingly, there is a need in the art to develop systems and methods to measure ammonia that are species selective, interference-free, relatively faster than prior art devices and methods, highly-resolved and affordable. Furthermore, there is a need to develop systems and methods that include a careful selection of optimum transitions that offer adequate sensitivity over the range of expected concentration and sufficient isolation from potential interfering species.
The present invention provides a system and method to measure the concentration of trace ammonia gas in the presence of large background carbon dioxide and water vapor populations. The present invention also provides a system and method to measure any combination of ammonia, carbon dioxide and water simultaneously.
The sensor system and method of the present invention for measuring ammonia in an environment containing carbon dioxide and water vapor include a single radiation source that is capable of spectrally interrogating an ammonia transition in an absorption band at substantially near 2 xcexcm (preferably 1993 nm). In this measurement the ammonia transition is substantially isolated from interfering absorption from the carbon dioxide and the water vapor. The ammonia transition at a frequency substantially close to 5016.977 inverse centimeters is selected in order to avoid interfering absorption from carbon dioxide and water vapor. For measuring ammonia only, a small scan by the radiation source would suffice.
The radiation source could also spectrally interrogate a carbon dioxide transition in an absorption band at substantially near 2 xcexcm, whereby the carbon dioxide transition would be substantially isolated from interfering absorption from ammonia and water vapor. The carbon dioxide transition that is used occurs at a frequency substantially close to 5017.030 inverse centimeters in order to avoid interfering absorption from ammonia and water vapor. Furthermore, the radiation source could also spectrally interrogate a water vapor transition in an absorption band at substantially near 2 xcexcm, whereby the water vapor transition would be substantially isolated from interfering absorption from ammonia and carbon dioxide. The water vapor transition that is used occurs at a frequency substantially close to 5017.100 inverse centimeters in order to avoid interfering absorption from ammonia and carbon dioxide.
The preferred radiation source is an infra-red single-frequency laser. An example of laser systems that could be used in the present invention are for instance, but not limited to, a semiconductor diode laser, a distributed feedback diode laser, a fiber-coupled distributed feedback diode laser, a fiber laser, or an optical parametric oscillator. In order to spectrally resolve the measurements of the different gas species various different techniques could be used. For instance, the system and method of the present invention could utilize scanned- and fixed wavelength absorption, balanced radiometric detection, frequency modulated (FM) spectroscopy, cavity-ring down, stark modulation, evanescent wave, photothermal deflection, optogalvanic spectroscopy or photoacoustic spectroscopy.
The system and method of the present invention further provide a means to operate at sub-atmospheric pressure to yield a good balance between signal strength and isolation of neighboring spectral transitions. The present invention teaches that a sub-atmospheric of substantially near 100 Torr (+/xe2x88x9220 Torr) would be optimal for measurements of ammonia. Using such a sub-atmospheric pressure decreases the pressure broadening of the different spectroscopic transitions, thereby isolating the different absorption features from each other, enabling species-specific measurements without interference from primary bath gas constituents.
As mentioned above, the sensor system and method of the present invention could also be used for measuring different gas species simultaneously with a radiation source (e.g. a tunable laser) that can sweep over the absorption transitions from all species simultaneously. In this case the sensor system and method of the present invention include a single radiation source that operates at a wavelength of substantially near 2 xcexcm (preferably 1993 nm) for simultaneously measuring a plurality of species along a single optical path in a gas mixture that contains the plurality of species. In this measurement, the absorption transitions that are interrogated for the plurality of species are proximate in frequency such that the radiation source can be scanned or stepped in the wavelength across the absorption transitions of all three species within a single measurement cycle. In the particular embodiment of the present invention, the plurality of species includes at least ammonia, carbon dioxide and water vapor. The present invention is not limited to the use of a single radiation source since it would be possible to include one or more additional radiation sources, for instance in a multiplexed fashion, each operating at a wavelength of substantially near 2 xcexcm. Each radiation source could interrogate one or more of the transition bands of ammonia, carbon dioxide and/or water vapor. The measurements parameters such as the pressure and temperature conditions are similar for a single and simultaneous measurement.
The present invention could be varied in several ways such as by including optical fibers for remote detection and measurement(s). The present invention could also be varied by providing optical fibers for remote detection or detection of multiple species and/or at multiple locations.
In view of that which is stated above, it is the objective of the present invention to utilize a radiation source to spectrally interrogate an ammonia transition in an absorption band at substantially near 2 xcexcm, whereby the ammonia transition is substantially isolated from interfering absorption from carbon dioxide and water vapor.
It is still another objective of the present invention to utilize a radiation source to measure NH3 at a frequency of 5016.977 inverse centimeters to avoid CO2 and H2O interference.
It is still another objective of the present invention to utilize a radiation source to measure CO2 at a frequency of 5017.030 inverse centimeters to avoid NH3 and H2O interference.
It is still another objective of the present invention to utilize a radiation source to measure NH3 and CO2 simultaneously with a scan that covers both of the aforementioned two objectives.
It is still another objective of the present invention to utilize a radiation source to measure H2O at a frequency of 5017.100 inverse centimeters to avoid CO2 and NH3 interference.
It is yet another objective of the present invention to measure NH3, CO2 and water vapor simultaneously with a single scan by the radiation source.
It is yet another objective of the present invention to utilize substantially near 100 Torr as the optimum measurement pressure for NH3 measurements to achieve a balance between highest achievable signal and narrowest spectroscopic transition. This optimum pressure is suitable for measurements of NH3 at any wavelength, not just 2 micron.
The present invention overcomes the limitations of prior art devices and methods and is characterized as species selective, interference-free, quick, highly-resolved and affordable. The present invention can be readily applied for in-situ measurements in certain measurement sites, including reduced pressure wafer etch chambers, before wafer damage occurs. The particular isolated ammonia transition at 2 xcexcm is more sensitive than the best wavelength at 1.5 xcexcm (where many optical sensors for ammonia operate), and is the strongest in the entire 2 xcexcm ammonia band that is isolated from both carbon dioxide and water vapor. The present invention takes advantage of lasers that operate near 2 xcexcm that have become commercially available in the last several years. Another advantage of a sensor that operates near 2 xcexcm is that standard telecommunications-grade low-OH silica optical fibers can be used in conjunction with the sensor for remote detection, multiplexing, and simultaneous measurement of multiple species and/or at multiple locations. Operating the measurement chamber at reduced pressure achieves better isolation between the target transition and neighboring locations. Though sub-atmospheric pressure is suitable in general, 100 Torr is the optimum pressure for measuring ammonia because that pressure achieves a balance between highest signal and narrowest, i.e. most isolated, transitions. Whereas for other species, for example CO2, 200 Torr is the optimum pressure for achieving highest signal with narrowest transition.